The Schwinger limit

Physicists are threatening to boil the quantum vacuum! The quantum vacuum is, purportedly, a seething foment of 'virtual' particles and anti-particles; each particle--anti-particle pair is created spontaneously from nothing, and mutually annihilates shortly thereafter. These particles are deemed virtual because they describe trajectories in space-time which violate the relativistic relationship between mass and energy-momentum; they are said to be 'off the mass-shell'.

However, if a force field can be created with a potential gradient greater than the Schwinger limit of 8 x 10¹⁸ volts per metre, then this will be sufficient to move the virtual particles onto the mass shell, and for the particle--anti-particle pairs to separate. In effect, these particles will begin to boil from the quantum vacuum. Some physicists are starting to think that high intensity laser technology will, within a few decades, be able to generate electromagnetic fields which exceed the Schwinger limit.